EPTGet fraps and your favourite video editing software (I used Camtasia Studios for this). Record yourself performing a simple task such as blinking stalkers on the unit creator map. Repeat a few times to smooth out the results. Then goto the video editing software, and find out how many frames passed between the mouse click and the frame the event happened. To figure out your latency, simply divide the number of frames passed by the number of frames recorded per second. Enjoy your newly found knowledge what your latency is.
For reference, I play from Israel on the European server with 1.5 Mbps ADSL line and I have
500 ms latency normally,
733 ms latency while downloading a file in the background,
and about 30 ms latency in single player (1 frame)
